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Live weight as a basis for deworming

Targeted selective treatment (TST) has been proposed as a sustainable method of gastrointestinal nematode control because it reduces the number of anthelmintic treatments administered, thereby preserving a susceptible nematode population in refugia. The most suitable criteria for selecting which animals to treat remains a subject of research. Irish researchers evaluated live weight as a basis for TST.



Over a two year period, weaned lambs from three farms were utilized for the study. On each farm, lambs were weighed and divided into heavy, medium and light weight classes. Within the heavy (n = 225) and light (n = 218) weight classes, lambs were randomly allocated to two treatments; anthelmintic treatment or no anthelmintic treatment. All lambs in the medium weight class were treated. Animal performance and parasitological parameters were assessed over a 28-day period.


Light lambs had significantly higher fecal egg count at the start of the trial than their heavy contemporaries, suggesting that the light lambs were more heavily parasitized; however, the same difference was not evident at day 28 when no significant difference in FEC was observed between the undosed heavy and light animals. Withholding anthelmintic treatment from lambs had a negative effect on worm egg count and animal performance with no evidence that heavy lambs were more resilient than light lambs when left untreated.


The proposition that live weight is a suitable criterion for identifying lambs that would benefit most from anthelmintic treatment was not supported in this research. The helminth fauna observed in sheep in Ireland constitutes primarily of Teladorsagia, Nematodirus, Trichostrongylus and Cooperia. Haemonchus is rare. Source: Veterinary Parasitology, July 2018.

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